Re-enable _LARGEFILE64_SOURCE when _GNU_SOURCE is defined

When we updated to the latest version of musl it broke some codebases
that were using LFS functions (e.g. `stat64`) and assuming those
functions would be defined when `_GNU_SOURCE` is defined.

See bminor/musl@25e6fee

This change effectively reverts the above one by defining
_LARGEFILE64_SOURCE whenever _GNU_SOURCE is defined.

This is what glibc does:

https://github.com/lattera/glibc/blob/895ef79e04a953cac1493863bcae29ad85657ee1/include/features.h#L206

system/lib/libc/musl/include/features.h

@@ -16,6 +16,11 @@
 #define _XOPEN_SOURCE 700
 #endif
 
+#if defined(_GNU_SOURCE)
+#undef _LARGEFILE64_SOURCE
+#define _LARGEFILE64_SOURCE 1
+#endif
+
 #if __STDC_VERSION__ >= 199901L
 #define __restrict restrict
 #elif !defined(__GNUC__)